Reset it to get a fresh installation of iOS 11. No need for battery replacement.
It's iOS 11 bugs causing battery drains or slowdowns.
It's a known thing that Apple slows down their devices with updates. It's not really for battery, it's for that you should be buying their latest iPhones. Battery story is funny.
They don't use memory cards, saying it slows down the performance, and then they themselves slow it down for lame reasons. Awesome strategies.